Organic chemistry (Amines)
An organic compound (A) with the molecular formula C8H9NO when treated with bromine in aqueous solution of sodium hydroxide gives a new compound (B) having the molecular formula C7H9N. The compound (B) on heating with chloroform and ethanolic potassium hydroxide forms a compound (C) having foul-smelling. Further, the compound (B) gives an alkali soluble sulphonamide on reacting with benzenesulfonyl chloride.
Moreover, a thermally unstable compound (D) is obtained when the compound (B) reacts with sodium nitrite in concentrated hydrochloric acid at very low temperature. Further, the compound (D) on heating with water yields a compound which gives colour with neutral ferric chloride solution.
Answer the following questions on the basis of this passage.
When compound (D) is heated with water, it gives:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 4-methylphenol (p-cresol)
From the passage, D is the diazonium salt of B, 4-methylbenzenediazonium chloride. Arenediazonium salts are stable only in cold solution. When the solution is warmed with water, the –\( \ce{N2^{+}} \) group is replaced by an –OH group and nitrogen gas escapes. The rest of the molecule, including the methyl group, is unchanged:
\( \displaystyle \ce{CH3C6H4N2^+Cl^- + H2O} \xrightarrow{\text{warm}} \ce{CH3C6H4OH + N2 ^ + HCl} \)
The product, 4-methylphenol (p-cresol), is a phenol. That is why it gives a colour with neutral ferric chloride solution, as the passage says. So the answer is 4-methylphenol.
Why the other options are incorrect
Option (2) benzene — Incorrect. Both the diazonium group and the methyl group would have to be removed; water does neither.
Option (3) toluene — Incorrect. Toluene forms when the diazonium group is replaced by hydrogen, which needs a reducing agent such as \( \ce{H3PO2} \) or ethanol, not water.
Option (4) phenol — Incorrect. Phenol has lost the methyl group, but the methyl group on the ring is not affected by this reaction.
NCERT (2026-27 edition):
Replacement by Hydroxyl Group
